It depends - there are several types of boats that serve WL/Fort Wilderness.
One type of boat is small and requires a step down to get into the boat. Those boats are only accessible to people who can get out of their wheelchairs, have the wheelchair folded and have a member of their party lift the wheelchair onto the boat.
The other type of boat is large and guests using wheelchairs and
ECVs can usually roll right on, unless the water levels are extremely high or extremely low. When that happens, the resorts use bus transportation as a back up.
There is more information in the disABILITIES FAQs thread or in
this thread.